When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. The body's autonomous nervous system generates intrinsic electrical heart activity signals that are conducted to atrial and ventricular heart chambers on the left and right sides of the heart. The electrical heart activity signals trigger resulting heart contractions that pump blood.
The intrinsic electrical heart activity signals can be monitored to provide an electrocardiogram (ECG) signal to a physician, clinician, diagnostician, or researcher to obtain information about heart function. In one such technique, a first external skin patch electrodes is adhesively affixed to the patient's right arm. A second external skin patch electrode is adhesively affixed to the patient's left arm. An instrumentation amplifier is used to detect the electrical heart activity signals at the first and second electrodes. The instrumentation amplifier outputs an ECG signal based on the difference of the signals at the first and second electrodes.
If no further electrodes are used, the ECG signal obtained between the first and second electrodes is typically severely degraded by common-mode (CM) noise signals, such as 60 Hertz or other environmental noise signals that are present at both of the first and second electrodes. Common-mode noise problems generally result even if a high-quality instrumentation amplifier is used. Skin-electrode interface impedance differences between the first and second electrodes contribute to such common-mode noise problems. Differences in skin-electrode interface impedances result from differences in body morphology, adhesion of the electrode, perspiration by the patient, etc. Because of the high input-impedance of the instrumentation amplifier, even small differences in the skin-electrode impedance (e.g., 10 kilo-ohms) can result in a common-mode noise signal amplitude that exceeds the amplitude of the desired ECG signal.
One technique of reducing the common-mode noise signal is to attach a third electrode, such as at the patient's right leg, for use in a feedback arrangement. The third electrode is driven by an offsetting common-mode signal to cancel a portion of the unwanted common-mode noise signal. However, this technique is inconvenient for the physician, because it requires attachment of the third electrode to the patient. This increases the complexity of the medical procedure. In a medical emergency, for example, such increased complexity is highly undesirable. Thus, there is a need for improved ECG measurement techniques providing adequate common-mode noise immunity without relying exclusively on attaching additional electrodes to the patient.